When speaking about the spatial scale of a band-dependent superconducting condensate, one usually means the characteristic length \(\xi _{\Delta j}\)  (the coherence length) of the gap function \(\Delta _j({\bf r})\) , where \(j\)  enumerates overlapping bands. Experimentally, \(\xi _{\Delta j}\)  can be extracted from the muon spin rotation measurements [1]}, from the scanning tunnelling microscopy (STM) results [2]}, [3]}, from the thermal conductivity of the vortex state [4]}, or from the upper critical field [5]}. However, one should keep in mind that relations between the gap-function distribution and the experimentally measured characteristics are generally model dependent, which complicates the interpretation of the experimental results. For example, the STM probes the local density of states (DOS) connected with the quasiparticle spatial distribution. The local relation between the zero-energy local DOS and the gap function is well-known only for the diffusive regime near the upper critical magnetic field [6]}. At lower fields the situation is significantly more complicated, especially in multiband superconducting materials [7]}, [8]}, [9]}.
